1. Field of the Disclosure
The present disclosure relates to an RF front-end module that performs mobile communication and to a wireless terminal having the RF front-end module.
2. Description of Related Art
In a conventional mobile communication system that uses a frequency division duplex (FDD) system in which a frequency domain to be used is divided into a domain for transmission and a domain for reception and transmission and reception are carried out concurrently, an uplink (UL) frequency band and a downlink (DL) frequency band have been fixedly paired. In U.S. Pat. No. 7,729,724 and U.S. Patent Application Publication No. 2002/0090974, for example, an RF front end used in an FDD system is disclosed. Conventional mobile communication systems of this type include, for example, a universal mobile telecommunications system (UMTS) and a frequency division duplex long term evolution (FDD-LTE).
FIG. 1 illustrates an example of three band pairs, B1, B3 and B4, as an example of pairs of frequency bands (band pairs) in an FDD system. Each band pair is a combination of a frequency for transmission (Tx) equivalent to uplink communication and a frequency for reception (Rx) equivalent to downlink communication.
By contrast, in a next-generation mobile communication system called LTE-Advanced, a technology called carrier aggregation (CA) is used. Carrier aggregation is a technology that combines a plurality of frequency carriers to expand frequency bands and improve communication speeds. In this technology, inter-band carrier aggregation is available which is an operation in which, for example, a frequency band for transmission (Tx) and a frequency band for reception (Rx) that constitute one band pair illustrated in FIG. 1 are combined with another frequency band.
When a plurality of frequency bands are combined as described above, not only can a wide frequency band be allocated but vertically asymmetrical operations become possible by combining split frequency bands or combining isolated frequency bands, enabling frequency bands to be efficiently used.
FIG. 2 illustrates an example of the structure of an existing RF front-end module that is ready for inter-band carrier aggregation.
The RF front-end module 50, which is placed between an antenna 101 and an RF transceiver 200, is a unit that performs filtering for out-of-band signals of RF signals, filter selection, and the like. The RF front-end module 50 internally includes a switch unit 10, a diplexer 20, and duplexers 131 to 134 (DUP#1 to DUP#4). The switch unit 10 includes RF switches 111 to 113.
The RF switch 111, which is placed between the diplexer 20 and the antenna 101, electrically connects or breaks a route between the diplexer 20 and the antenna 101 by being controlled to be turned on and off. The RF switch 112, which is placed between the duplexer 133 and the antenna 101, electrically connects or breaks a route between the duplexer 133 and the antenna 101 by being controlled to be turned on and off. The RF switch 113, which is placed between the duplexer 134 and the antenna 101, electrically connects or breaks a route between the duplexer 134 and the antenna 101 by being controlled to be turned on and off.
The diplexer 20 functions so as to split a frequency band to be used between the duplexer 131 and the duplexer 132.
Since, in a FDD system, transmission and reception are performed concurrently, the duplexers 131 to 134 each function so as to provide an isolation between the transmission (Tx) frequency band and the reception (Rx) frequency band, which are paired, so that a transmission (Tx) signal and a reception (Rx) signal do not mutually interfere.
As illustrated in FIG. 3, the duplexer 131 is formed with, for example, a band-pass filter (transmission BPF) 412 that selectively passes signals in the transmission (Tx) frequency band, a band-pass filter (reception BPF) 413 that selectively passes signals in the reception (Rx) frequency band, and a phase circuit (phase shifter) 411 connected in series with the reception BPF 413. The phase circuit 411 is structured (designed) so that a mutual relationship between frequency characteristics of the two filters becomes appropriate in the setting of a matching circuit and line length. This prevents transmission signals and other unnecessary signals from entering the reception side, enabling transmission and reception signals to be preferably split. The matching circuit may be generally formed with an inductor or a capacitor. The phase circuit 411 may be provided not only on the same side as the reception BPF 413 but also on the same side as the transmission BPF 412.
Referring again to FIG. 2, the RF transceiver 200 has power amplifiers 211 to 214, which are respectively connected to the duplexers 131 to 134. The input ends of the power amplifiers 211 to 214 are respectively transmission ports #1 to #4 of a known transmission circuit (not illustrated) provided in the RF transceiver 200. The RF transceiver 200 also has reception ports #1 to #4, which are respectively connected to the duplexers 131 to 134. Known reception circuits (not illustrated) that include low-noise amplifiers (LNAs) 221 to 224 are connected to the reception ports #1 to #4.
FIG. 4 is a graph used to explain the action of a duplexer. The horizontal axis of the graph represents frequencies [MHz], and its vertical axis represents the amount of signal attenuation [dB]. A waveform Wt indicates frequency characteristics that represent how a signal proceeding from the transmission port of the duplexer to an antenna terminal is attenuated. A waveform Wr indicates frequency characteristics that represent how a signal proceeding from the antenna terminal of the duplexer to a reception port is attenuated.
An area A1 of the waveform Wt indicates a low insertion loss in a transmission frequency band, indicating that the efficiency of transmission electric power is superior. An area A2 of the waveform Wr indicates a low insertion loss in a reception frequency band, indicating that reception sensitivity is superior.
An area A3 of the waveform Wr indicates that a wraparound from the transmission signal to the reception side is suppressed by the duplexer and deterioration in reception sensitivity due to the transmission signal is thereby suppressed. Similarly, an area A4 of the waveform Wt indicates that it is suppressed by the duplexer that noise generated from the power amplifier enters the reception side and reception sensitivity is thereby deteriorated.
In the structure illustrated in FIG. 2, when a single band pair (one transmission frequency band and one reception frequency band) is used, the RF switch 112, which is connected to the duplexer 133 connected to the transmission port #3 and reception port #3, for example, is turned on and the other RF switches are turned off. Alternatively, the RF switch 113, which is connected to the duplexer 134 connected to the transmission port #4 and reception port #4, is turned on and the other RF switches are turned off.
In the structure in FIG. 2, if an operation is performed in inter-band carrier aggregation of two downlink waveforms (2 DL) and one uplink (1 UL), the RF switch 111 connected to the diplexer 20 is turned on and the other RF switches are turned off. At this time, the transmission port #1 is used for transmission and the reception ports #1 and #2 are used for reception, for example.